The present invention relates to a superconductive logic circuit of current coupling type which does not involve a magnetic flux quantum interferometer and, more particularly, to a superconductive logic circuit comprised of Josephson junctions, which may be used to constitute various kinds of logic circuits, which has a high current gain and which has a wide operation margin.
Conventional superconductive logic circuits are roughly classified by the input system into the magnetic coupling type and the current injection type. In a superconductive logic circuit of magnetic coupling type, an input signal is magnetically coupled to a loop including a Josephson junction and inductance, and logic operations are performed by transformation of the Josephson junction to the non-zero voltage state as shown in U.S. Pat. No. 3,978,351. In this example, the Josephson junction and the inductance constitute a magnetic flux quantum interferometer, and the product L.IJ of the inductance L and the critical current IJ of the Josephson junction is selected to be close to one magnetic flux quantum .phi.0. Therefore, when the critical current IJ is made small for energy consumption, a large inductance L is required, making it difficult to realize a compact circuit and reducing the operating speed. Conversely, when the inductance L is made smaller for obtaining a higher operating speed, the value of the critical current IJ becomes greater and energy consumption increases. Further, the circuit is subject to the influence of external magnetic noise, stray inductance and so on, resulting in extreme fluctuations and unstable operation. Such a circuit is also defective in that uniform and efficient connection of a number of input wires is difficult structurally.
A superconductive logic circuit of current injection type has been proposed as an improvement over such a circuit, which does not involve a magnetic flux quantum interferometer. In a superconductive logic circuit of current injection type, current is directly supplied to the Josephson junction for switching into the non-zero voltage state in order to perform logic operations. A prior art superconducting logic circuit of the current injection type which does not involve a magnetic flux quantum interferometer is shown in IEDM "Josephson Direct Coupled Logic (DCL)" (1492, 12), IBM. According to this example, the defects of the superconductive logic circuit of the magnetic coupling type which involve magnetic flux quantum interferometer are solved. However, the threshold for determining the sensitivity is solely determined by the switching of the non-zero voltage state of a single Josephson junction so that only a current gain of at most 1 may be obtained. Therefore, although it is advantageous to use it as a switch, it is difficult to apply it to various kinds of logic circuits.
The object of the present invention is to provide a superconductive logic circuit which eliminates the defects of the prior art circuits, which has a high sensitivity, therefore, a high gain and a wide operation margin, and which is advantageous for assembly into various kinds of logic circuits.